The B-X electronic system of 79 Br 2 , 79,81 Br 2 , and 81 Br 2 has been investigated with laser-induced fluorescence at a resolution corresponding to 1% of the Doppler width. Hyperfine-free frequency differences between pairs of nearly coincident rovibronic transitions have been measured to an accuracy of Β±7 MHz. The data fall into two categories: cases where the two members of the pair arise from different isotopomers, and cases where they arise from the same isotopomer. From the frequency differences, determinations have been made of the electronic isotope shifts arising from terms in the Hamiltonian that lead to the breakdown of the Born-Oppenheimer approximation. In the Born-Oppenheimer approximation, the electronic term value T BO BX for the system B 3 0+u -X 1 + g is isotopically invariant. However, the measurements show that [T 79 BX -T 79,81 BX ] = 177(10) MHz; [T 79,81 BX -T 81 BX ] = 209(7) MHz; and [T 79 BX -T 81 BX ] = 386(7) MHz. In the next order of approximation, [T Ξ± BX -T Ξ² BX ] should be proportional to (1/Β΅ Ξ± -1/Β΅ Ξ² ), where Β΅ Ξ± is the reduced mass of isotopomer Ξ±. The measured values of [T Ξ± BX -T Ξ² BX ] are seen to deviate from this dependence by approximately 15%. It is suggested that the deviation occurs because the Hamiltonian for the heteronuclear isotopomer includes interactions that are forbidden for the homonuclear species.